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In systems which make photographic emulsions utilizing silver ion concentrations, an ion sensor senses the concentration of ion in the mixture and provides a signal which is a function of the concentration. A desired concentration for the mixture is generally represented as a set point signal which is compared against the concentration signal from the ion sensor to provide a difference signal. This difference signal is used to control the flow rate of silver salt and halide salt solutions flowing into a precipitation vessel. It is well-known that the signal from the ion sensor is non-linear, which translates into having to provide large amounts of silver nitrate to the emulsion to cause relatively small voltage changes in the sensor's output when the sensor is operating in one range of its output versus having to add slight amounts of additional silver nitrate to cause the same magnitude of voltage change in the sensor's output when the sensor is operating in another range of its output.
A prior art patent of interest for its teachings in this art area is U.S. Pat. No. 3,999,048 entitled "Flow Control System for the Precipitation of Silver Halide Emulsions" by K. G. Parthemore. The invention of that patent directs its focus on solving the problem of a non-linear sensor output being due to the non-linear characteristics of the measured variable (silver ion concentration) in the precipitating process which results in unwanted over or under conditions in the control of the valves that interrupt the flow of the silver salt and halide salt into the precipitation vessel. The invention appreciates the non-linearity of the sensor's output and through an anti-log circuit provides a degree of linearity to that signal to provide an improved mixing system.
It is believed, that because of the resolution limits of the silver ion measurement sensor that the concentration measurement in the form of a voltage can only be accurate within a limited range. Therefore, feedback control based on a concentration voltage results in a degree of over and/or under control in the actual precipitation.
Improvements in the field of ion concentration controllers, therefore, are possible by improving the linearization of the ion sensor's output signal. The present invention is directed towards such an improvement.